Nucleic acid sequence analogues, in particular tRNA analogues comprising in the place of a 3'-terminal adenosine a 3'-terminal 2'-deoxy-2'-thioadenosine are able to react site-specifically with weakly activated amino acid derivatives (e.g. phenylthioesters of amino acids), thereby connecting the amino acid to the 2'-thiol-group of the 3'-terminal thioadenosine via a thioester bond. A fast and thermodynamically favored intramolecular transesterification reaction leads then to the formation of the corresponding 3'-O-acyl-derivative. A correspondingly modified tRNA is an active analogue of an aminoacylated tRNA and allows introduction of the amino acid bound thereto into proteins. The aminoacylated tRNA analogue according to the invention can be isolated in pure form under acidic conditions (pH 5.0). At pH 7.4 and 25 DEG C it is found to have a half-life of 25 min, which compares well with the half-life of 40 min for the corresponding native compound.